The present invention relates to a method for adhesively bonding two bodies together, at least one of which is metallic, and articles produced by this method.
A need has long existed for a method of enhancing the adhesive bonding characteristics of various materials by pretreatment of the surfaces of materials to be bonded. This need has extended to enhancing the bonding characteristics of materials which are essentially clean surfaces, e.g., solvent cleaned surfaces, as well as enhancing the bonding characteristics of contaminated surfaces, e.g., surfaces covered with a protective coating such as a wax or oil coated surface.
These needs have developed for example out of the automotive and aircraft industries' dissatisfaction with current pretreatment techniques and bonding results.
One means of treating metals for specific benefits such as surface hardening, surface smoothing, welding and hole drilling is the use of an energy beam such as a laser beam. An elegant explanation of metal treatment using energy beams comprising laser beams is contained in U.S. Pat. No. 4,122,240 granted to Banas et al. FIG. 1 therein shows a plot of absorbed power density versus interaction time. At relatively low absorbed power density applied for relatively long times, materials can be heated to appreciable depths without melting the surface and thus can be subjected to "transformation hardening." At very high absorbed power density applied for extremely short periods of time to a surface coated with a thin layer of preferably black paint (which enhances absorption of the laser beam) surface vaporization of the paint is so violent that a shock wave of sufficient amplitude moves through the material that can result in "shock hardening" (see U.S. Pat. No. 4,401,477 to Clauer et al.). At mid-absorbed power density applied for intermediate time periods, the irradiated material can be vaporized to a significant depth for "hole drilling" applications. At lower power density applied for longer time periods, the material can be melted to significant depths for "deep penetration welding." Using approximately the same absorbed power density as in "deep penetration welding," but for shorter periods of time, Banas et al. achieved "skin melting." In skin melting a thin layer of the material irradiated is melted but not vaporized and then rapidly self-cooled.
Langen et al. in U.S. Pat. No. 4,368,080 apply 0.5 to 16 joules/cm.sup.2 per pulse and a pulse time of from 1 to 100 microseconds to clean rust from metallic objects prior to painting. The power density used by Langen et al. is relatively low to prevent melting or vaporization of the parent metal, but high enough to vaporize the rust or to convert the rust to a form not detrimental to subsequent paint performance.
In Japanese Patent Application No. 54-26509 to Iuchi et al., a laser is used to remove oil from steel plates prior to painting. The oil is compounded with light absorbing chemicals to improve the efficiency of conversion of the light energy into heating the oil film and only the oil is vaporized, not the underlying steel.
In Japanese Patent No. 81,116,867 zinc galvanized steel sheets are polished to remove the zinc coating, irradiated with a laser beam to remove residual zinc, phosphate treated and then painted.
Japanese Patent Application No. 59-85392 to Nippon Avionics, describes a method for binding a metallic substrate to a metallic material or a non-metallic material. The method comprises irradiating a laser beam on the surface of the metallic substrate in air or a selected gas, thereby melting the irradiated portion of the substrate and forming projections on the surface of the substrate, coating an adhesive on the so-treated surface and adhering it to a metallic or non-metallic material. This reference teaches that generally it is desirable to make projections more than 20 micrometers in height, 50 micrometer projections being easily made, to produce a better bond than produced using conventional pretreatments such a sandblasting.
Prior techniques for enhancing the bonding characteristics of metallic bodies, exclusive of energy beam treatment, include sandblasting, shot peening, brushing, pickling with acid, anodizing and washing with solvents, see for example, "Adhesives Technology Handbook," A. H. Landrock, 1985, Noyes Publications, ISBN 0-8155-1040-3. All of these techniques have undesirable features such as waste disposal of spent chemicals. 5Nevertheless, surface treatment usually results in enhanced bondability. For example, galvanized steel sheets formed into automotive door panels are generally coated with a lubricant prior to forming to extend forming die life and to preserve the surface finish of the panel. However, before the panel can be fastened to other parts with an adhesive, the lubricant usually needs to be removed from the panel, e.g., by washing with a solvent, for yet improved bondability. Often, even better bondability can be obtained by additionally roughening or etching the panel surface to be bonded by sandblasting and/or chemical treatment, such as a phosphate treatment. The present invention is an improved method of the laser treatment type.